T cells play a central role in virtually all types of immune responses. These cells trigger antibody production, initiate delayed-type hypersensitivity, generate killer cells, and in some instances, exert immunosuppressive effects. Our recent studies represent the first direct demonstration that these responses are not mediated by a single class of pluripotent T cells. Rather, this class is divisible into several subclasses, each of which is equipped to mediate a limited range of immune functions, each distinguishable by expression of different surface Ly markers. Thus, cells capable of generating killer, helper, and suppressor function are likely to fall into distinct Ly plus subclasses. These findings open a new areas of research which has important theoretical and practical implications. For example, studies of the relationships among the Ly plus T-cell subclasses will yield new information concerning the differentiative process resulting in the formation of immunocompetent T cells. Insight provided from these studies into the patterning of normal T cell differentiation will provide a basis for investigation of the clinical difficulties encountered in both transplantation of organs and bone marrow cells.